Code refactorisation for the structure.align user function backend.
The looping over data pipes, model numbers, and molecule names, skipping those that don't match the
function arguments, has been shifted into the new structure_loop() generator function of the
pipe_control.structure.main module. This function assembles the data from the data store and then
calls the new loop_coord_structures() generator function of the lib.structure.internal.coordinates
module which does all of the work.
